The present invention relates to a method to form microlithographic images using a projection exposure system for fabricating semiconductor devices.
Semiconductor lithography involves the creation of small three dimensional features as relief structures in a photopolymeric or photoresist coating. These features are generally on the order of the wavelength of the ultraviolet (UV) radiation used to pattern them. Currently, exposure wavelengths are on the order of 150 to 450 nm and more specifically 157 nm, 293 nm, 248 nm, 365 nm, and 436 nm. The most challenging lithographic features are those which fall near or below sizes corresponding to 0.5xcex/NA, where xcex is the exposing wavelength and NA is the objective lens numerical aperture of the exposure tool. As an example, for a 248 nm wavelength exposure system incorporating a 0.60NA objective lens, the imaging of features at or below 0.25 micrometers is considered state of the art. FIG. 1 shows the configuration of a projection exposure system. Such an exposure system can be used in a step-and-repeat mode (referred to a stepper tool) or in a step-and-scan mode (referred to as a scanner tool). A UV or vacuum ultraviolet (VUV) source 1 is used to pass radiation through the illumination system 2 using a condenser lens system 3 and a fly""s eye microlens array 4. An aperture 5 shapes the illumination profile to a defined area and radiation is reflected from a mirror 6 to pass through an illumination lens 7 to illuminate a photolithographic mask 8. Upon illumination of the photomask 8, a diffraction field 11 distributed as spatial frequency detail of the photomask 8 is directed through the objective lens system 9 to be imaged onto the photoresist coated semiconductor substrate 10. Such an exposure system forms an image by collecting at least more than the 0th-order of the diffraction field from the photomask 8 with the objective lens 9. The absolute limitation to the smallest feature that can be imaged in any optical system corresponds to 0.25xcex/NA. Furthermore, the depth of focus (DOF) for such an exposure tool can be defined as +/xe2x88x92 k2xcex/NA2 where k2 is a process factor that generally takes on a value near 0.5
As geometry sizes continue to shrink further below 0.5xcex/NA, methods of resolution enhancement are being required to ensure that intensity images (also known as aerial images) are produced with adequate fidelity and captured within a photoresist material. Such methods of resolution enhancement developed over recent years can allow for improvement in addition to those made possible with shorter exposing wavelengths and larger numerical apertures. These methods in effect work to control the weighting of diffraction energy that is used for imaging. This diffraction energy corresponds to the spatial frequency detail of a photomask. Phase-shift masking (PSM), off-axis illumination (OAI), and optical proximity correction (OPC) all lead to image enhancement through control of the weighting of diffraction energy or spatial frequency that is collected by an objective lens. As an example, an attenuated phase shift photomask accomplishes a phase shift between adjacent features with two or more levels of transmission [see B. W. Smith et al, J. Vac. Sci. Technol. B 14(6), 3719, (1996)]. This type of phase shift mask is described for instance in U.S. Pat. Nos. 4,890,309 and 5,939,227. Radiation that passes through clear regions of the such a mask possess a phase (xcfx86) that is dependant on the refractive index and thickness of the mask substrate. Radiation is also Transmitted through dark features formed in the attenuating material by proper choice of a material that has an extinction coefficient value generally less than 1.0. The radiation that passes though these dark features possesses a phase that depends upon the refractive index and extinction coefficient values of the mask substrate and of the attenuating material. It is chosen so that a 180 degree phase shift (xcex94xcfx86) is produced between clear regions and dark regions. Selection of a masking material with appropriate optical properties to allow both a 180 degree phase shift and a transmission of some value greater than 0% will reduce the amplitude of the zero diffraction order produced by illumination of the mask. Comparison of the resulting frequency plane distribution with that of a conventional binary mask can demonstrate this effect. The normalized zero diffraction order amplitude for a binary mask is 0.5 and the first order amplitude is 1/xcfx80 as seen from FIG. 2A. Using a 10% attenuated phase shift mask, the normalized zero diffraction order amplitude is 0.4 and the first order amplitude is 1.1/xcfx80 shown in FIG. 2B. This reduction in the zero diffraction order reduces the amplitude biasing of the higher order frequency components and produces an image amplitude function that has significant negative electric field energy. This leads to air aerial image intensity (which is the square of the amplitude image) that retains zero values at edges of opaque features. This edge sharpening effect leads to higher resolution when imaging into high contrast photoresist materials.
An attenuated phase shift mask requires a complex infrastructure of materials, deposition, etching, inspection, and repair techniques to replace the mature chromium on quartz binary photomask process. This field has been investigated for over ten years and it is not yet certain if suitable materials will exist for 248 nm, 193 nm, or shorter wavelengths. Stronger phase shift masking is difficult because of geometry, materials, and process issues and diffractive and scattering artifacts produced during imaging. As a result, it is questionable how practical phase shift masking will be for use in integrated circuit (IC) manufacturing.
Off-axis or modified illumination of a photomask can produce a similar frequency modifying effect [see B. J. Lin, Proc. SPIE 1927, 89, (1993) and see B. W. Smith, Microlithography: Science and Technology, Marcel Dekker: New York, Ch. 3, 235 (1998)]. FIG. 3 shows an example of the prior art, depicting illumination with an annulus and quadrupole illumination profiles. Definition of the shape of illumination can be carried out for instance in the position of the shaping aperture 5 shown in FIG. 1. Other methods of shaping can include the use of beam splitters, diffractive optical elements, or other optical approaches. Through the us, of such annular or quadrupole illumination, diffraction orders can be distributed in the objective lens 9 of FIG. 1 with minimal sampling of the central portion of the objective lens pupil. An example, is shown in FIGS. 4a through 4c for annular, quadrupole, and weak quadrupole illumination. The impact is similar to the reduction of the zero diffraction order or frequency produced with phase shift masking. In this case, the zero order takes on the shape of the illumination distribution in the condenser lens pupil. If appropriately designed, the central portion of the objective lens pupil can be avoided by diffraction energy.
Modified or off-axis illumination can suffer exposure throughput, orientation, and proximity effect problems. Contact mask features, for example, exhibit little improvement with off-axis illumination. Implementation is therefore limited for many applications, also limiting practicality.
The use of optical proximity correction (OPC) can also result in a reduction of zero order diffraction energy in the frequency plane. Methods of proximity effect reduction have been introduced which are comprised of additional lines, sometimes referred to as OPC assist features, into a mask pattern. This was first disclosed in U.S. Pat. No. 5,242,770. The patterning is such that an isolated line is surrounded by sub-resolution OPC assist line features on either side of the line, better matching edge intensity gradients of isolated features on the mask to more dense features on the mask. FIG. 5A shows the frequency plane distribution in the objective lens pupil for semi isolated lines (1:7 duty ratio) compared to the frequency plane distribution of dense lines (1:1 duty ratio), FIG. 5B. In addition to the increase in the number of diffraction orders present for the more isolated features (resulting from a larger pitch value than that for the dense features) a significant increase in the zero order term exists. Through the use of small assist features on each side of the isolated line, the amplitude of the zero diffraction order can be reduced as higher frequency content is increased, seen in FIG. 5C for one pair and for two pairs of assist features respectively.
OPC methods are limited by mask making capability for ultra small geometry and by limitations imposed by neighboring geometry. Implementation is especially difficult for geometry below 180 nm in size, limiting practicality.
Direct reduction of zero order diffraction energy within the lens and specifically in the lens pupil can be carried out by physically obscuring the central axial portion of the pupil. The concept of using such iii-pupil filters (also known as pupil-filtering) has been applied to various optical applications, where the result is a spatial frequency filtering effect. This has also been studied by various workers for application to semiconductor lithography (see W. T. Welford, J.O.S.A., Vol. 50, No. 8 (1960), 749 and see H. Fukuda, T. Terasawa, and S. Okazaki, J. Vac. Sci, Tech. B 9 (1991) 3113 and see R. M. von Bunau, G. Owen, R. F. Pease, Jpn. J. Appl. Phys., Vol. 32 (1993) 5850.). This in-pupil filtering has also been proposed in patents U.S. Pat. No. 5,595,857, U.S. Pat. No. 5,863,712, U.S. Pat. No. 5,396,311, and U.S. Pat. No. 5,677,757. The imaging characteristics for fine geometry (generally features at or below 0.5xcex/NA) have been shown to be enhanced through the use of various pupil filters. A simple example of the prior art is shown in FIG. 6, where a pupil filter 38 has a radiation-blocking portion 39 that blocks zero-order diffraction energy from passing through a central area of the filter and a radiation or radiation or light transmitting portion 40 that transmits diffraction energy at a peripheral area surrounding the radiation or light blocking portion 39. With respect to the projection imaging system of FIG. 1, such a prior art filter 14 is inserted into the pupil plane of the objective lens of the exposure system. By obscuring the central portion of the objective lens pupil, zero diffraction order energy is reduced. Opaque or partially transmitting (gray) obscuration of up to 70% of the pupil as well as more complex pupil filters have also been used where the spatial frequency filtering of the filter is customized for specific illumination and masking situations to meet certain imaging objectives. Implementation of such in-pupil filtering for semiconductor lithography is not practical or feasible because access to the objective lens pupil is difficult given the strict requirements placed on the objective lens performance. The filtering or obscuration of the objective lens pupil requires access to the pupil and a lens design robust enough to tolerate any phase, absorption, or flatness variations. It is unlikely that a permanent filtering value would be chosen for any lithography lens.
A practical solution of spatial frequency filtering is needed that can lead to the resolution and focal depth improvement that is difficult with other resolution enhancement methods. The ideal solution is one that could preserve most attributes of current manufacturable methods of lithography and allow the flexibility for application with many applications. Furthermore, a spatial frequency filtering solution that could be used together with other resolution enhancement methods could reduce demands on those methods to allow for their application.
The present invention is a unique approach to reducing zero diffraction order energy by spatial frequency filtering, in an alternative pupil plane, near the mask or the wafer planes and accessible to the user in a conventional lithography system. A conventional binary mask can be used with this approach. A conventional full circular pupil can be also used as the diffraction information is not filtered within the lens pupil. Furthermore, if combined with phase shift masking, modified illumination, or optical proximity correction. further improvement becomes feasible.
A goal of the presents invention is to provide a practical projection exposure method and apparatus that is capable of providing image improvement for a variety of fine lithographic features, including one-dimensional and two-dimensional geometry, in terms of both resolution and depth of focus by reducing the amount of zero order diffraction energy that is translated from a photomask and through the objective lens of a stepper or scanner system.
A second goal of the present invention is to provide a practical projection exposure method and apparatus that is capable of improving the performance of small contacts or via openings that are otherwise difficult to image using other resolution enhancement techniques.
A further goal of the present invention is to provide a practical projection exposure method and apparatus that is capable of providing means to improve the image forming characteristics of features that are different in their optical performance from one another so that images in photoresist can be created that are most favorable for all features at a single exposure value or across a small range of exposure.
An additional goal of the present invention is to provide a practical projection exposure method and apparatus that is capable of providing imaging improvement that can be implemented in an exposure system without significant modification to the system.
A still further goal of the present invention is to provide a practical projection exposure method and apparatus that is capable of providing for custom image modification based on the mask feature type, mask type, illumination, or other imaging condition and can be easily inserted or removed from an exposure system.
A still further goal of the present invention is to provide a practical projection exposure method and apparatus that does not introduce significant, measurable, or uncorrectable sources of error to an exposure tool, including uncontrollable effects from aberration, distortion, polarization, and defocus.
A still further goal of the present invention is to provide a practical projection exposure method and apparatus that can be specifically designed according to desired image forming properties of a mask feature type, mask type, illumination, or other imaging condition.
A still further goal of the present invention is to provide a practical projection exposure method and apparatus that allows for imaging improvement at one or more locations outside of the objective lens, where a single location or multiple locations may be used during imaging.
In order to accomplish the above described goals, spatial frequency filtering is carried out with the present invention at locations outside of the objective lens and at planes alternative to the objective lens pupil. This allows access to diffraction field energy or frequency information without requiring access to the physical lens pupil. Alternative pupil planes are located close to object and image planes, or mask and semiconductor substrate planes respectively. At predetermined distances from these positions, separation is sufficiently far so that angular specific spatial frequency filtering devices placed at these locations will selectively act on frequency content of an image rather than spatial content (i.e. positional or distance). A filter is provided that controls transmission as a function of angular properties of the diffraction field from the photomask geometry. The filter consists of coatings of predetermined transparent materials at predetermined thicknesses on a transparent substrate so that normal or near normal incident radiation passes through the filter attenuated by a predetermined amount while radiation at more oblique angle passes unattenuated or weakly attenuated. The angular transmission properties of the filter act on the spatial frequency content of the photomask patterns and resulting images, where zero order diffraction field energy is attenuated to the largest degree.
Additionally, the present invention provides a method to hold the filtering device or devices at the predetermined separation distances. One aspect of the present invention provides a spacing element which consists of a fixed ring of a width corresponding to the required separation distance which is fixed to both the photomask and to the filter, holding the filter at the required separation distance. Alternatively, the projection exposure apparatus is provided with an optical changing member for selecting one of a number of a plurality of filters that provide preferred image modification for at least one optical characteristic of the photomask features. Alternatively, the filter is secured at the bottom most position of the objective lens column, closest to the image plane of the exposure system.